It is hard to determine whether the air quality is good or bad by our senses alone, because some air pollutants, like carbon monoxide, are colourless and odourless. Even air that looks clean may contain a lot of harmful substances. Air quality indices are tools to compute and indicate air quality. Nowadays, many cities announce their own ‘Air Pollution Index” or “Air Quality Health Index”, but in fact are using different data and calculations so we cannot directly compare one index with another.
If we take Hong Kong’s former “Air Pollution Index” as an example, we may discover it converts the concentrations of several major air pollutants to a scale between 0 and 500. The pollutant with the highest index is regarded as determining the final “Air Pollution Index”. The “Air Pollution Index” is also classified as one of the five levels – Low, Medium, High, Very High and Severe. Hong Kong had been using the “Air Pollution Index” since 1995. This made it easier for the public to get a clear picture of our air quality, and raised everyone’s concern regarding air quality.
Because the “Air Pollution Index” shifts to measure and compare the concentrations of air pollutants, it apparently ignores the relationship between these pollutants and health risks. Also, “Air Pollution Index” is computed on a 24-hour basis, so cannot reflect the air quality at this moment or for a particular couple of hours. In some places such as Canada, “Air Pollution Index” has been gradually replaced by “Air Quality Health Index”.
In Hong Kong, the “Air Quality Health Index” became effective in December 2013. The index is calculated from status of four pollutants: nitrogen dioxide, ozone, sulphur dioxide and air particulates. It computes the average concentration of these pollutants in the last three hours. Each pollutant is linked to a risk factor, with a value proportional to the health impact. With the input of risk factors, concentrations of the four pollutants add up to form the “Air Quality Health Index”. By contrast with the old “Air Pollution Index”, which only represented the concentration of one pollutant with the highest index, the “Air Quality Health Index” includes four types of pollutants, so is more comprehensive and representative. More importantly, the new index incorporates health risks, so is more practical and compatible with our needs.